Muscular strength in male adolescents and premature death: cohort study of one million participants

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Dr. Leckstrőm and Dr. Goldsmith raised a couple of issues concerning our recently published study (1). First, they argued that “participants with mental infirmity or chronic conditions would not have been permitted to perform these tests; while many eligible men might deliberately have underperformed/underachieved so as to avoid the possibility of military involvement”. Also, they indicated that “…it would be very important to understand more about low strength group (1st decile) (its history, and future trajectory), before assuming that corrective interventions at this early stage could prove beneficial decades later”

It is important to note that adolescents with severe handicap or a severe chronic disease were exempted from the conscription examinations, and therefore are not included in the analyses. Apart of them, conscription examinations were mandatory by law for all young male Swedish citizens, and they predate active military service; that is, they are also done for boys who do not later enter military service (2). In addition, to have a low level of strength level was not an exclusion criterion to enter military service, so that deliberately underperforming for this purpose seems unlikely. We observed that the low strength group (1st tenth) had by far the highest risk. This is in fact a major finding of this study, a very low muscular strength is good marker to identify “unhealthy” people with a substantially increased risk for premature death. Our results are consistent with those from previous cohorts, for example, adult men from the Aerobic Center Longitudinal Study (ACLS) showed a 28% risk difference in all-cause mortality between the lowest and middle thirds of muscular strength, while there was very little difference in the risk of all-cause mortality between the middle and upper thirds of muscular strength (3). The consistency between our findings and those from previous studies suggest that having high levels of muscular strength is not associated with substantially further risk reduction in mortality, compared to middle levels of muscular strength. This issue is discussed in the manuscript (third paragraph of the Discussion).

The second issue concerns how generalizable these findings are for adolescents from other countries. Dr. Leckstrőm and Dr. Goldsmith argue that “Sweden is far from typical in its habits, so one should caution against too ready an extrapolation of these findings to other countries”. However, when looking at the WHO-Europe health statistics (http://data.euro.who.int/hfadb/), we can see that Sweden is rather typical country in Europe. If we leave apart the countries from the former Eastern Bloc or Communist Bloc (generally Soviet Union and countries of the Warsaw Pact), which have by far the highest mortality rates and lower life expectancy, Sweden is right in the middle of the European ranking for cardiovascular disease mortality (age-standardized rates per 100,000 persons aged 0-64 years=28.8) and life expectancy (81 years).

Ortega and his Nordic colleagues report the findings of a study which examined how muscular strength (knee extension, handgrip and elbow flexion) measured in adolescent Swedish men typically aged 16-18 years, conducted at Swedish Defence Recruitment Agency subscription assessments between 1969 and 1994, impacted on subsequent mortality (1). They found that high muscular strength in adolescence was associated with a 20-35% reduction in all-cause and cardiovascular mortality (1), independent of the effects of body-mass index and blood pressure. This Swedish male cohort has been used before reporting on mortality impacts of blood pressure and body mass index (2,3).

Impressive outputs from this analysis include links not only to cardiovascular outcomes (though not co-adjusted for aerobic fitness), but also to the future development of mental illness, and death by suicide. The effect size of the association between muscular strength and mortality was the same as those seen for body mass index and hypertension (2,3).

The main debate for us seems to centre on two considerations. First, just how easy is it to disentangle the discrete and relevant impacts of body weight, muscle strength, aerobic fitness, and personal “motivation” on the overall performance these young male potential recruits achieved ? Those with mental infirmity or chronic conditions would not have been permitted to perform these tests; while many eligible men might deliberately have underperformed / underachieved so as to avoid the possibility of military involvement.

From the results presented in the paper there appeared to be a “lowest performing decile” with the greatest risk of future mortality but it would be very important to understand more about this group (its history, and future trajectory), before assuming that corrective interventions at this early stage could prove beneficial decades later.

Our second issue is to ponder on just how generalisable these findings might be to adolescent men (let alone women, rarely recruited, and not included here) from other countries, without the social cohesion and other societal habits of Sweden? In its use of a conscription-based army (from 1901 to 2010), despite a carefully-maintained, and often subtly-nuanced, national neutrality (4), substantial tobacco consumption using Snus (5), strict control of alcohol distribution through the Systembolaget (6), and, their historical, and current, relative lack of obesity and diabetes compared to most other European countries (7,8), Sweden is far from typical in its habits, so one should caution against too ready an extrapolation of these findings to other countries. What might work well for the Sami, in Skåne, or in Stockholm, might mis-fire in Sardinia, Skopje, or Santander.

The findings presented in this paper are intriguing, and available uniquely because of the Swedish societal system. We think that a case could be made for a randomised controlled trial of a carefully calibrated “intervention” for 16-18 year olds whose attainments in these sorts of assessments are particularly unimpressive. But which priorities should be set - a focus on aerobic fitness, muscular strength, BMI, smoking, or mental health / self-esteem ? Or somehow to try to target all of these at once ?

Daniel CT Leckstrőm (3rd Year Medical Student King’s College School of Medicine and Dentistry, London)

David JA Goldsmith (Reader in Renal Medicine, King’s College School of Medicine and Dentistry, London)